This invention relates to electric power converters and, more particularly, relates to an isolated and soft-switched power converter having two resonant tank circuits coupled back-to-back through an isolation transformer. The invention also relates to electric power storage and generation systems and electric vehicles using the converter.
Soft switching techniques have been used in power converters to reduce switching losses and alleviate electromagnetic interference (EMI). For example, soft switching techniques can be particularly important in electric power applications involving DC/AC and DC/DC power conversion and relatively large power requirements, such as electric vehicle, hybrid electric vehicle, and electric power storage and generation systems. In electric vehicle applications, for example, the power controller for the electric motor uses frequency control, phase control, pulse control, and other types of power supply manipulation to smoothly control the power output of the electric motor. This type of power supply manipulation requires a high rate of switching in the power controller to generate the precisely-controlled power supply waveforms to drive the electric motor.
If this switching occurs in the power controller when current is flowing through or voltage is applied across the switching element (i.e., xe2x80x9chard switchingxe2x80x9d), the resulting switching losses in the power controller decrease the efficiency of the vehicle""s power plant, the high voltage rise rate (dV/dt) may damage the motor, and the resulting voltage change rate can cause additional losses and overheating in the electric motor, as well as interfering with the operation of other electrical systems. Switching with zero current flowing through or zero voltage across the switching element, which is known as xe2x80x9csoft switching,xe2x80x9d alleviates these problems. Obtaining soft-switching can be difficult, however, because it requires repeatedly driving the voltage across or forcing the current through the switching element to zero, and then holding the voltage or current at this zero level long enough for the switching element to physically switch. Because an AC voltage wave periodically passes through a zero-voltage state, but does not remain at the zero-voltage state for an extended period, the switching element may not have enough time to physically switch while the voltage transitions through the zero-voltage state.
The need to switch during zero-voltage and/or zero-current states or transitions sets the stage for the basic soft switching design objectives. The voltage across each switching element should repeatedly obtain zero-voltage and/or zero-current periods long enough for the element to soft switch, or looked at from the other direction, the switching elements must be capable of switching fast enough to soft switch during the zero-voltage and/or zero-current transitions. As another design concern, the cost of the switching elements increase with increasing power transmission capability and switching speeds.
Electric vehicle or hybrid electric vehicle designs also have other important objectives, including physical and electrical isolation of high-voltage circuits and components (e.g., the electric drive system) from the low-voltage system. Another important objective is bidirectional operation of the power converter so that the automotive battery, which is used to start the vehicle, can be recharged during vehicle operation. Electric or hybrid electric vehicles typically use a relatively low-voltage DC automotive electrical system (e.g., about 12 Volts or 36 Volts), and a relatively high-voltage DC electrical system for the electric motor that drives the vehicle (e.g., about 300 Volts). Thus, the typical electric or hybrid electric vehicle application calls for a bidirectional DC/DC power converter with high-voltage isolation. Certain load-side electric generation applications, such as battery storage peak shaving, also require bidirectional DC/DC power conversion.
Because DC voltage cannot be increased without intermediate conversion to an alternating voltage, and the underlying automotive battery operates at a relatively low DC voltage, the DC/DC power converter for an electric or hybrid electric vehicle implements the steps of DC/AC conversion at low voltage, AC/AC voltage boost through an AC transformer, and AC/DC conversion at high-voltage. Accordingly, the DC/AC and AC/DC conversion steps involve switching in the inversion and rectification processes which, if uncontrolled, can generate large switching losses and EMI. In addition, subsequent inversion of the high-voltage DC output, typically in the power controller or inverter for the electric motor, can generate large switching losses and EMI.
Conventional bidirectional DC/DC power converters developed for hybrid electric vehicle and other applications have a number of disadvantages. Specifically, two separate full-bridge converters are needed to utilize the full DC voltage in the power conversion. That is, a first full-bridge converter is needed for the DC/AC conversion, and a second full-bridge converter is needed for the subsequent AC/DC conversion. This requires an excessive number of components in the duplicate full-bridge converters. In addition, transformer leakage can result in voltage surges, power losses, and control difficulties.
Furthermore, some conventional DC/DC power converter designs require a clamping or snubber circuit to provide a DC current path through an inductor for limiting voltage surge during switching. These circuit designs also experience hard-switching related problems, such as high EMI and high voltage rise rates, which require sophisticated filters and shielding. Moreover, for traditional DC/DC/AC power conversion, these circuit designs require two separate stages (DC/DC and DC/AC power conversion), which further duplicate parts without synergy.
In general, conventional DC/DC and DC/DC/AC power converters typically provide an output voltage that is equal to or less than the voltage available from the internal DC power supply. For safety considerations, manufacturers of electric drive systems in products for general distribution, such as electric vehicles, would prefer to have power distribution busses operate at no more than 50 Volts. But lower voltage motors are inherently larger and heavier than higher voltage motors delivering equivalent power, because lower voltage motors must be made with copper wire that is large enough to safely handle the higher current required for equivalent power delivery at a lower voltage.
These conflicting design objectives create serious design constraints and unresolved problems for the designers of power converters. Thus, there is a need in the art for improved DC/DC and DC/DC/AC converters for many applications, including hybrid electric vehicles and electric utility applications, such as remote load-side electric power generation. In particular, there is a need for a soft switched power converter that isolates high-voltage components, uses conventional switching devices, exhibits stable voltage control, alleviates EMI production, and avoids unnecessary duplication of components.
The present invention meets the needs described above in an isolated and soft-switched power converter for DC/DC and DC/DC/AC power conversion. The power converter utilizes the full DC voltage of internal voltage sources in both the positive and negative polarities for intermediate AC/AC transformation, with a minimum number of switching devices and other electric components. The power converter also repeatedly maintains zero-voltage periods across its switching elements to allow low-stress soft switching by conventional switching elements. That is, the power converter provides extended zero-voltage switching periods to allow soft switching by conventional switching devices, rather than relying on very fast switching during very short or transient zero-voltage periods. These attributes provide the advantages of economic construction and ready-availability of the required components.
The power converter produces bidirectional DC/DC and DC/DC/AC power conversion, and also produces a quasi-DC output voltage that repeatedly obtains sustained zero-voltage periods for soft switching by subsequent components, such as an electric vehicle power controller. The power converter also provides electrical and physical isolation of high-voltage components, soft-switching to all internal switching devices, bidirectional power flow between the input and output, and good voltage control. In addition, alternative designs may further minimize the number of components, for example in a unidirectional power converter, and by relying on stray capacitance in switching and clamping devices and/or leakage and self inductance in the isolation transformer in lieu of discrete capacitors and/or inductors.
In an electric or hybrid electrical vehicle application, the power converter solves the competing design objectives of a high-voltage drive motor and a low-voltage distribution system by physically and electrically isolating the low-voltage distribution system from the high-voltage electric drive. For example, the low-voltage automotive electrical system typically operates at about 12 Volts DC, 36 Volts DC, or both, whereas the high-voltage drive electric typically operates at about 300 Volts DC. In load-side electric generation applications, the high-voltage side may operate at 240 or 480 Volts for industrial/commercial backup systems, and at 12 or 25 kV for utility generators, such as peak load shaving and energy storage equipment.
The invention also provides these solutions in a cost-effective manner, using a minimal number of conventional switching devices. In an electric vehicle application, the power converter invention allows a low-voltage distribution system to supply power to an adjustable speed drive that incorporates an isolated boost converter to output medium- to high-voltage AC power with soft switching of all devices. Further, the inverter can be designed so that it is enclosed with the drive motor so that the isolated high-voltage components are never exposed outside the case.
The circuit configuration of the power converter includes two resonant tank circuits coupled back-to-back through an isolation transformer. Each resonant tank circuit may consist of a pair of resonant capacitors connected in series as a resonant leg, a pair of tank capacitors connected in series as a tank leg, and a pair of switching devices with anti-parallel clamping diodes coupled in series as resonant switches and clamping devices for the resonant leg. Thus, the power converter uses far fewer switching devices as compared with the traditional bidirectional isolated power converters. As a result, the power converter is well suited for DC/DC and DC/DC/AC power conversion applications in which high-voltage isolation, DC to DC voltage boost, bidirectional power flow, and a minimal number of conventional switching components are important design objectives. For example, the power converter is especially well suited to electric and hybrid electric vehicle applications, load-side electric generation and storage systems, and other applications in which these objectives are important.
Generally described, the invention includes an isolated and soft-switched power converter including two resonant tank circuits coupled back-to-back through an isolation transformer. Each resonant tank circuit includes a pair of resonant capacitors present in series as a resonant leg, a pair of tank capacitors connected in series as a tank leg, and a pair of switching devices coupled in series as resonant switches and voltage clamping device for the resonant leg. The power converter also includes a switching controller operable for gating the switching devices to cause a resonant voltage to resonate in each resonant leg. In addition, the resonant voltage repeatedly obtains zero-voltage periods for soft-switching a device powered by the converter. The switching controller also gates the switching devices during zero-current and/or zero-voltage conditions for soft switching the switching devices of each resonant tank circuit.
The invention also includes a hybrid electric/combustion engine vehicle including the power converter described above. The hybrid vehicle includes an automotive battery electrically connected to the primary resonant tank circuit, and a low-voltage automotive electrical system electrically connected to the automotive battery. The hybrid vehicle also includes a combustion engine, an electric motor, and a power controller connected to the secondary resonant tank circuit. The power controller provides a controlled power input to the electric motor in response to an accelerator signal received from an operator of the vehicle. A mechanical transmission driven by the combustion engine and the electric motor rotationally drives wheels to transport the vehicle. In addition, the power converter operates to deliver electric power from the automotive battery to the electric motor during acceleration and relatively low speed vehicle transportation or as determined by the vehicle""s operational strategy controller. The power converter also operates to deliver electric power from the electric motor to the automotive battery during deceleration and during periods of low propulsion power demands.
In another alternative, the invention includes a fuel-cell powered electric vehicle including the power converter. The fuel-cell automobile includes a battery electrically connected to the primary resonant tank circuit and a low-voltage automotive electrical system electrically connected to the automotive battery. The fuel-cell automobile also includes a fuel cell, a compressor, and a traction-drive motor with their respective power controllers electrically connected to the secondary resonant tank circuit.
The traction motor""s power controller provides controlled power input to the electric motor in response to an accelerator signal received from an operator of the vehicle. A mechanical transmission driven by the electric motor rotationally drives wheels to transport the vehicle. In addition, the compressor""s power converter operates to deliver electric power from the low-voltage automotive battery to the high-voltage compressor during start-up of the fuel cell. The power converter also operates to deliver electric power from the fuel cell to the automotive battery during operation of the fuel cell.
For example, in a typical configuration, the automotive electrical system may operate at about 12 Volts DC or about 36 Volts DC, the fuel cell may operate at about 300 Volts DC, and the power converter may operate at a resonant frequency in the range of about 10 kHz to 100 kHz.
In another application, the invention includes an electric storage and generation system using the power converter. The system includes a battery storage unit connected to the low side of the power converter, and an inverter/rectifier connected to the high side of the power converter. The inverter/rectifier, in turn, connects the system through a conventional transformer to an electric power grid. In an other alternative, the battery is charged through a remote generator, such as a photovoltaic (PV) panel. In this case, the power flow may be unidirectional into the power grid. As another option, the PV panel or another type of remote DC power generator may be connected directly to the low side of the power converter.
More specifically described, the power converter includes an isolation transformer having a primary input node, a primary output node, a secondary input node, and a secondary output node. The power converter also includes a primary resonant tank circuit having a primary top rail, a primary center rail, and a primary bottom rail. In addition, the primary resonant tank circuit includes a first primary resonant capacitance present between the primary top rail and the primary center rail, a second primary resonant capacitance present between the primary center rail and the primary bottom rail, and a first primary resonant switch connected between the primary top rail and the primary center rail.
The primary resonant tank circuit also includes a second primary resonant switch connected between the primary center rail and the primary bottom rail, a first primary clamping diode connected between the primary high-voltage rail and the primary center rail, and a second primary clamping diode connected between the primary center rail and the primary bottom rail. The primary resonant tank circuit further includes a first primary tank capacitor connected between the primary top rail and a primary tap node, and a second primary tank capacitor connected between the primary tap node and the primary bottom rail. In addition, the primary center rail is connected to the primary input node of the isolation transformer, and the primary tap node is connected to the primary output node of the isolation transformer.
The power converter also includes a secondary resonant tank circuit including a secondary top rail, a secondary center rail, and a secondary bottom rail. The secondary resonant tank circuit includes a first secondary resonant capacitance present between the secondary top rail and the secondary center rail, a second secondary resonant capacitance present between the secondary center rail and the secondary bottom rail, and a first secondary clamping diode connected between the secondary top rail and the secondary center rail. The secondary resonant tank circuit also includes a first secondary tank capacitor connected between the secondary top rail and a secondary tap node, and a second secondary tank capacitor connected between the secondary tap node and the secondary bottom rail. The secondary center rail is connected to the secondary input node of the isolation transformer, and the secondary tap node is connected to the secondary output node of the isolation transformer.
The secondary resonant tank circuit of the power converter may also include a second secondary clamping diode connected between the secondary center rail and the secondary bottom rail. The secondary resonant tank circuit may also include a first secondary resonant switch connected between the secondary top rail and the secondary center rail, and a second secondary resonant switch connected between the secondary center rail and the secondary bottom rail.
The power converter may also include a primary bottom-rail terminal connected to the primary bottom rail for connection to a low-potential terminal of a DC voltage source, and a primary center-rail terminal for connection to a high-potential terminal of the DC voltage source. The power converter may also include an inductor connected in series between the primary center-rail terminal and the primary center rail. Alternatively or additionally, the power converter may include a resonant inductor connected between the primary input node of the isolation transformer and the primary output node of the isolation transformer.
The power converter also includes a switching controller operable for gating the first and second primary switches to cause a primary resonant voltage to resonate between the first primary resonant capacitance and the second primary resonant capacitance, and a secondary resonant voltage to resonate between the first secondary resonant capacitance and the second secondary resonant capacitance. The secondary resonant voltage repeatedly obtains zero-voltage periods for soft-switching a device connected between the secondary center rail and the secondary bottom rail. In addition, the switching controller is operable for soft switching the first and second primary resonant switches by gating the first resonant switch during current conduction by the first clamping diode, and by gating the second resonant switch during current conduction by the second clamping diode.
In one configuration, the first primary resonant capacitance includes a discrete electrical capacitor connected between the primary top rail and the primary center rail, and the second primary resonant capacitance includes a discrete electrical capacitor connected between the primary center rail and the primary bottom rail. In an alternative configuration, the first primary resonant capacitance consists essentially of stray capacitance inherently present in the first primary resonant switch and the first primary clamping diode, and the second primary resonant capacitance consists essentially of stray capacitance inherently present in the second primary resonant switch and the second primary clamping diode.
Similarly, the first secondary resonant capacitance may include a discrete electrical capacitor connected between the secondary top rail and the secondary center rail, and the second secondary resonant capacitance may include a discrete electrical capacitor connected between the secondary center rail and the secondary bottom rail. Alternatively, the first secondary resonant capacitance may consist essentially of stray capacitance inherently present in the first secondary clamping diode and the second secondary resonant capacitance may consist essentially of stray capacitance inherently present in the second secondary clamping diode.
The invention also includes a hybrid electric and combustion engine powered vehicle including the power converter described above. This hybrid vehicle includes an automotive battery having a high-potential terminal electrically connected to the primary center-rail terminal of the power converter. The battery also has a low-potential terminal electrically connected to the primary bottom-rail terminal of the power converter. The hybrid vehicle also includes a low-voltage automotive electrical system electrically connected to the automotive battery, a combustion engine, and an electric motor. The power controller is connected to the secondary resonant tank circuit, and provides a controlled power input to the electric motor in response to an accelerator signal received from an operator of the vehicle. The hybrid vehicle also includes a mechanical transmission driven by the combustion engine and the electric motor. The transmission rotationally drives wheels to transport the vehicle in response to rotational power delivered by the combustion engine and the electric motor. In addition, the power converter operates to deliver electric power from the automotive battery to the electric motor during acceleration and relatively low speed vehicle transportation or as determined by the vehicle""s operational strategy controller. The power converter also operates to deliver electric power from the electric motor to the automotive battery during deceleration and during periods of low propulsion power demands.
The power controller in the hybrid vehicle may include a variable-frequency inverter connected across the secondary center rail and the secondary bottom rail. In this case, the variable-frequency inverter receives a quasi-DC supply voltage, which allows the inverter to soft-switch during the zero-voltage periods occurring between the secondary center rail and the secondary bottom rail. In a hard-switching alternative, the power controller includes a variable-frequency inverter connected across the secondary top rail and the secondary bottom rail. In this case, the variable-frequency inverter receives a relatively constant DC supply voltage.
The invention also includes a fuel-cell powered electric vehicle including the previously-described electric power converter. This fuel-cell powered vehicle includes an automotive battery having a high-potential terminal electrically connected to the primary center-rail terminal of the power converter. The battery also has a low-potential terminal electrically connected to the primary bottom-rail terminal of the power converter. The fuel-cell powered vehicle also includes a low-voltage automotive electrical system electrically connected to the automotive battery. The fuel-cell powered vehicle also includes a fuel cell electrically connected across the secondary top rail and the secondary bottom rail of the secondary resonant tank circuit, and a compressor electrically connected across the secondary top rail and the secondary bottom rail of the secondary resonant tank circuit. This compressor is configured to deliver compressed gas, such as air or a hydrogen and air mixture, to the fuel cell.
The fuel-cell powered vehicle also includes an electric traction motor and a power controller connected to the secondary resonant tank circuit and providing a controlled power input to the electric traction motor in response to an accelerator signal received from an operator of the vehicle. A mechanical transmission driven by the electric motor is configured to rotationally drive wheels to transport the vehicle. In addition, the power converter operates to deliver electric power from the automotive battery to the compressor during start-up of the fuel cell. The power converter also operates to deliver electric power from the fuel cell to the automotive battery during operation of the fuel cell.
The power controller in the fuel-cell powered vehicle may include a variable-frequency inverter, connected across the secondary center rail and the secondary bottom rail, which soft-switches during the zero-voltage periods occurring between the secondary center rail and the secondary bottom rail. Alternatively, the power controller may include a variable-frequency inverter connected across the secondary top rail and the secondary bottom rail. That the invention improves over the drawbacks of prior art power converters and accomplishes the advantages described above will become apparent from the following detailed description and the appended drawings and claims.